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JSP的本质是什么？

zhaoxj0217 2022-08-03 阅读 70
jspjavatomcatapacheSpark大数据


概述

  1. JSP页面在本质上就是Servlet程序，当JSP页面在首次被访问时，Ｗeb容器就会将JSP页面转化为Servlet，只需要做一次。如index.jsp在首次被访问时，Ｗeb容器会将其翻译成一个index_jsp.java文件，即Servlet代码；
  2. Servlet程序要被WEB容器调用执行，必须在web.xml中注册映射；
  3. Servlet类继承了类org.apache.jasper.runtime.HttpJspBase(Tomcat API,Tomcat容器实现的),这个类实现了HttpJspPage, JspPage, Servlet, ServletConfig等这些接口，因此WEB容器必须实现这些接口。

WEB容器： Tomcat、Jetty、Apache、Nginx、Jboss、Undertow、Websphere、Kangle、Resin等。

二、重要API

Servlet API：​​https://tomcat.apache.org/tomcat-8.5-doc/servletapi/index.html​​​ Tomcat API：​​https://tomcat.apache.org/tomcat-8.5-doc/api/index.html​​ JSP API：​​http://tomcat.apache.org/tomcat-8.5-doc/jspapi/index.html​​

三、jsp页面转化为servlet程序

（１）index.jsp：

<html>
<body>
<h2>
<%
    out.println("Hello Tomcat");
%>
</h2>
</body>
</html>

（２）index.jsp在首次被访问时，WEB容器将jsp转化为Servlet类，即index.jsp转化为index_jsp.java：

JSP的本质是什么？_jsp

index_jsp.java:

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/8.5.47
 * Generated at: 2019-11-07 08:52:49 UTC
 * Note: The last modified time of this file was set to
 *       the last modified time of the source file after
 *       generation to assist with modification tracking.
 */
package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;

public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent,
                 org.apache.jasper.runtime.JspSourceImports {

  private static final javax.servlet.jsp.JspFactory _jspxFactory =
          javax.servlet.jsp.JspFactory.getDefaultFactory();

  private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;

  private static final java.util.Set<java.lang.String> _jspx_imports_packages;

  private static final java.util.Set<java.lang.String> _jspx_imports_classes;

  static {
    _jspx_imports_packages = new java.util.HashSet<>();
    _jspx_imports_packages.add("javax.servlet");
    _jspx_imports_packages.add("javax.servlet.http");
    _jspx_imports_packages.add("javax.servlet.jsp");
    _jspx_imports_classes = null;
  }
 private volatile javax.el.ExpressionFactory _el_expressionfactory;
  private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager;

  public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
    return _jspx_dependants;
  }

  public java.util.Set<java.lang.String> getPackageImports() {
    return _jspx_imports_packages;
  }

  public java.util.Set<java.lang.String> getClassImports() {
    return _jspx_imports_classes;
  }

  public javax.el.ExpressionFactory _jsp_getExpressionFactory() {
    if (_el_expressionfactory == null) {
      synchronized (this) {
        if (_el_expressionfactory == null) {
          _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
        }
      }
    }
    return _el_expressionfactory;
  }
 public org.apache.tomcat.InstanceManager _jsp_getInstanceManager() {
    if (_jsp_instancemanager == null) {
      synchronized (this) {
        if (_jsp_instancemanager == null) {
          _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
        }
      }
    }
    return _jsp_instancemanager;
  }

  public void _jspInit() {
  }

  public void _jspDestroy() {
  }

  public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
      throws java.io.IOException, javax.servlet.ServletException {

    final java.lang.String _jspx_method = request.getMethod();
    if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
      response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD");
      return;
    }

    final javax.servlet.jsp.PageContext pageContext;
    javax.servlet.http.HttpSession session = null;
    final javax.servlet.ServletContext application;
    final javax.servlet.ServletConfig config;
    javax.servlet.jsp.JspWriter out = null;
    final java.lang.Object page = this;
    javax.servlet.jsp.JspWriter _jspx_out = null;
 javax.servlet.jsp.PageContext _jspx_page_context = null;


    try {
      response.setContentType("text/html");
      pageContext = _jspxFactory.getPageContext(this, request, response,
                        null, true, 8192, true);
      _jspx_page_context = pageContext;
      application = pageContext.getServletContext();
      config = pageContext.getServletConfig();
      session = pageContext.getSession();
      out = pageContext.getOut();
      _jspx_out = out;

      out.write("<html>\n");
      out.write("<body>\n");
      out.write("<h2>\n");

    out.println("Hello Tomcat");

      out.write("\n");
      out.write("</h2>\n");
      out.write("</body>\n");
      out.write("</html>\n");
 } catch (java.lang.Throwable t) {
      if (!(t instanceof javax.servlet.jsp.SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          try {
            if (response.isCommitted()) {
              out.flush();
            } else {
              out.clearBuffer();
            }
          } catch (java.io.IOException e) {}
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
        else throw new ServletException(t);
      }
    } finally {
      _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }
}

从转化后的代码，可知jsp页面里的HTML、CSS、JS，最终都被嵌套在Servlet程序的java代码里。我们可以推断jsp页面只是方便我们编写Servlet程序，就像在拼接一个长长的字符串。但是在前后端分离的开发中（可以参考​​《前后端分离的趋势》​​），jsp显然是不适应的。

四、分析index_jsp.java类

  • 注释部分

/*
 * Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/8.5.47
* 　　...
*/

注释里说*.jsp文件都会被Apache Tomcat容器的Jasper组件转化为Servlet的子类。再次证明，jsp程序的本质就是Servlet程序。

  • Servlet类的继承关系

public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent,
                 org.apache.jasper.runtime.JspSourceImports {

HttpJspBase实现了Serializable, HttpJspPage, JspPage, Servlet, ServletConfig这些接口。而HttpJspBase是Tomcat API，所以Tomcat容器实现了这些。

  • 内置对象out、request、response、session、application
    我们分析index_jsp.java的_jspService方法来学习这些内置对象。

public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
      throws java.io.IOException, javax.servlet.ServletException {

    final java.lang.String _jspx_method = request.getMethod();
    if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
      response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD");
      return;
    }
    final javax.servlet.jsp.PageContext pageContext;
    javax.servlet.http.HttpSession session = null;
    final javax.servlet.ServletContext application;
    final javax.servlet.ServletConfig config;
    javax.servlet.jsp.JspWriter out = null;
    final java.lang.Object page = this;
    javax.servlet.jsp.JspWriter _jspx_out = null;
 javax.servlet.jsp.PageContext _jspx_page_context = null;
    try {
      response.setContentType("text/html");
      pageContext = _jspxFactory.getPageContext(this, request, response,
                        null, true, 8192, true);
      _jspx_page_context = pageContext;
      application = pageContext.getServletContext();
      config = pageContext.getServletConfig();
      session = pageContext.getSession();
      out = pageContext.getOut();
      _jspx_out = out;

（１）request
request是_jspService方法的参数，类型为HttpServletRequest，专门用来获得客户端向服务端请求的信息。
（２）reponse
reponse是_jspService方法的参数，类型为HttpServletResponse，专门用来填充要发送给客户端的数据。
（３）out、session、application
_jspxFactory对象调用 getPageContext()方法返回 PageContext对象。再通过调用PageContext对象的相应方法来初始out、session、application这三个内置对象。

private static final javax.servlet.jsp.JspFactory _jspxFactory =
          javax.servlet.jsp.JspFactory.getDefaultFactory();

从上面的代码可知：

out是一个 JspWriter对象。JspWriter对象与ServletResponse的PrintWriter关联着，所以你会发现不用直接使用reponse对象，也可以往response对象写数据。
JspWriter的方法请参考：​​​http://tomcat.apache.org/tomcat-8.5-doc/jspapi/index.html​​

session是一个 HttpSession对象。用来维护会话期间用户的信息。关闭浏览器此会话就会被关闭。
HttpSession的方法请参考：​​​https://tomcat.apache.org/tomcat-8.5-doc/servletapi/index.html​​

application是一个 ServletContext对象。它的生命周期与程序一样长，只有程序被关闭，它才会消亡。
ServletContext的方法请参考：​​​https://tomcat.apache.org/tomcat-8.5-doc/servletapi/index.html​​

小知识：HTML、CSS、JS的处理是由客户端的浏览器来完成的，WEB服务器只需要将这些资源返回给浏览器即可，动态部分都是组装好再发送的。

谢谢阅读！


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